Abstract

An isograd represents a line on a map resulting from the intersection of an isogradic surface with the topography. It is inferred to represent a metamorphic reaction and is inherently diachronous because of the time required to move heat, fluid, and reactions through the rocks. Relief of 500 m or more or reconstruction of post-metamorphic faulting and folding will be essential to estimate the orientation of an isogradic surface. Ductile rocks require that isobaric surfaces dip gently during metamorphism, and they must do so in restored sections. Isotherms can be constructed into cross-sections because the angle between the isograds and isotherms are a function of the P–T slope of univariant reaction curves. Near Mica Creek, British Columbia, we can infer an 1100-bar difference in pressure along a kyanite–sillimanite (ky–sil) isograd, based upon structural relief. The experimentally determined ky–sil P–T curve suggests a P/T slope of ~20 bars/°C. We calculate the apparent variation in P and T along the isograd and reconstruct the isotherm geometry. Dehydration-reaction curves, at relatively high P, have steep P–T slopes, and the associated isogradic surfaces can approximate isothermal surfaces. Some dehydration isograds are “smeared out”, but for regional scales, these latter isogradic surfaces at high P still approximate the isothermal surface geometry. Data on isogradic surfaces can be used to set limits on parts of the P–T grids for pelitic rocks. Locally, bathozone-defining isobaric surfaces can be reconstructed; we suggest a ΔP of 2 kbar for the garnet – biotite – kyanite bathozone.